1. Field of the invention
The present invention relates to a backlight system and a driving method for the same, and more particularly to a low-noise back light system for use in TFT (thin film transistor) LCD device and a driving method for the same.
2. Description of the Prior Art
As generally known in the art, a fluorescent lamp is used for many applications in which light is needed, but power for generating the light is limited. Such applications include a backlight system for use in a flat panel computer display. One of special types of the fluorescent lamp is a cold cathode fluorescent lamp (CCFL). A CCFL tube generally contains argon gas, Xenon gas, etc., together with a small amount of mercury. After an initial spark and a generation of plasma, alternating current flows through the CCFL tube and then ultra-violet rays are generated. Ultra violet rays radiate onto a fluorescent layer coated on an inner wall of the tube, thereby creating visible lights.
A CCFL inverter serves to receive direct current voltage from an outer power source and to supply alternating current to the CCFL tube, thus making the CCFL tube illuminated. As the modes for modulating brightness of the CCFL tube, a conventional CCFL inverter includes a pulse width modulation dimming mode and an analogue dimming mode. Of these two modes, the pulse width modulation dimming mode is used to generate driving current (alternately, driving voltage) by making use of PWM signal, a pulse width of which is modulated depending on a magnitude of the current flowing through the CCFL, and to supply the driving current. In contrast with the analogue dimming mode, a CCFL tube being operated in the pulse width modulation dimming mode repeats a process which comprises a turn-on with 600 to 800 volts and a turn-off. Thus, much noise and voltage fluctuation can arise in power voltages (typically 12 V) of the CCFL inverter. The noise and the voltage fluctuation can affect the whole driving circuit including an analogue unit and a logic unit, thus leading to deterioration of the display on LCD panel.
Also, in the conventional CCFL inverter, a frequency of the pulse width modulation (PWM) dimming mode has been separately designed from a vertical synchronization signal V_sync of a display device. Accordingly, upon an occurrence of interference between the PWM signal and the vertical synchronization signal, there has been a problem in that a horizontal wave rises in the LCD device. In order to solve this problem, a method for preventing the horizontal wave from being generated has been proposed, in which the inverter includes a phase locked loop circuit for synchronizing the PWM signal with the vertical synchronization signal. However, such a method has another problem in that because the phase locked loop circuit is sensitive to noises, if a number of CCFL tubes are installed, the high noise and voltage fluctuation occur in the power voltage and the CCFL tubes cannot work properly, as described above. Also, in the case that a phase locked loop circuit is employed, the lamp may be overloaded because the frequency of lamp driving power varies as the vertical synchronization signal varies among 60 Hz, 70 Hz, 75 Hz, etc.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a backlight system which makes it possible to reduce noises and voltage fluctuations due to the turning on and turning off of the lamp and a method for driving such a backlight.
Another object of the present invention is to provide such a backlight system which makes it possible to eliminate a horizontal wave or a flicker due to interference between the PWM signal and vertical synchronization signal and a method for driving a backlight.
Another object of the present invention is to provide a backlight system which makes it possible to drive a lamp with a constant frequency even when a vertical synchronization signal varies and a method for driving a backlight.
In order to accomplish these objects, according to the present invention, there is provided a backlight system including two lamps, in which a power supply unit for supplying the lamps with alternating voltage or alternating current supplies power for driving the lamps to each lamp with a predetermined time lag or phase difference. With this feature of the present invention, the magnitude of noise and voltage fluctuation occurring in the power voltages supplying to the power supply unit may be significantly reduced. Thus, deterioration of display quality, flicker, etc., due to noise and voltage fluctuation can be prevented. Also, there is provided a backlight system which may produce a lamp driving signal with a constant frequency obtained by multiplying a frequency of vertical synchronization signal by an integer. With this feature of the present invention, a horizontal wave or a flicker can be easily eliminated by producing the lamp driving signal synchronized with the vertical synchronization signal, and operation of the lamp can be stabilized as well because the lamp is driven by a lamp driving power with a constant frequency.
According to an aspect of the present invention, the low noise backlight system for use in a display device comprises: first and second lamps; a control unit for receiving a vertical synchronization signal of the display, producing a first control signal which has a duty cycle controlled depending upon currents flowing in the first lamp and is synchronized with the vertical synchronization signal, and producing a second control signal which has a duty cycle controlled depending upon currents flowing in the second lamp, is synchronized with the vertical synchronization signal and has a predetermined time lag with respect to the first control signal. The backlight system comprises a first power supply unit for causing the first lamp to be driven in synchronization with the first control signal and supplying the first driving voltage to the first lamp. Also, it comprises a second power supply unit for producing a second driving voltage for causing the second lamp to be driven in synchronization with the second control signal and supplying the second driving voltage to the second lamp.
The control unit comprises a first frequency multiplier for multiplying a frequency of the vertical synchronization signal by an integer to produce a first pulse width modulation frequency signal; and a signal delayer for delaying the first pulse width modulation frequency signal for a predetermined time to produce a second pulse width modulation frequency signal. Also it comprises a first current measuring unit for measuring currents flowing in the first lamp to produce a first feedback signal; and a second current measuring unit for measuring currents flowing in the second lamp to produce a second feedback signal. Also it comprises a first pulse width modulator for producing a first pulse width modulation signal which is synchronized with the first pulse width modulation frequency signal and has a duty cycle determined in accordance with the first feedback signal; a second pulse width modulator for producing a second pulse width modulation signal which is synchronized with the second pulse width modulation frequency signal and has a duty cycle determined in accordance with the second feedback signal. Further, it comprises a first control signal generator for receiving the first pulse width modulation signal, measuring the frequency thereof, and producing the first control signal with a constant frequency obtained by an integer multiplication depending on the measured frequency; and a second control signal generator for receiving the second pulse width modulation signal, measuring the frequency thereof, and producing the second control signal with a constant frequency obtained by an integer multiplication depending on the measured frequency.
The first control signal generator comprises: a first frequency detecting circuit for receiving the first pulse width modulation signal and measuring a frequency thereof; and a first frequency multiplication circuit for producing the first control signal with a constant frequency obtained by multiplying the first pulse width modulation signal by an integer depending on the measured frequency of the first pulse width modulation signal. The second control signal generator comprises a second frequency detecting circuit for receiving the second pulse width modulation signal and measuring a frequency thereof; and a second frequency multiplication circuit for producing the second control signal with a constant frequency obtained by multiplying the second pulse width modulation signal by an integer depending on the measured frequency of the second pulse width modulation signal.
The first power supply unit comprises: a first switch which is turned on by the first control signal and outputs power voltages through its output terminal; and a first transformer including a first coil connected to the output terminal of the first switch and a second coil connected to the first lamp. The second power supply unit comprises: a second switch which is turned on by the second control signal and outputs power voltages through its output terminal; and a second transformer including a first coil connected to the output terminal of the second switch and a second coil connected to the first lamp.
According to the other aspect of the present invention, a method for driving a backlight system including a first and a second lamp for use in a display device comprises steps of: receiving a vertical synchronization signal of the display device, producing a first control signal which has a duty cycle controlled depending upon currents flowing in the first lamp and is synchronized with the vertical synchronization signal, and producing a second control signal which has a duty cycle controlled depending upon currents flowing in the second lamp, is synchronized with the vertical synchronization signal and has a predetermined time lag with respect to the first control signal. Also the method comprises steps of producing a first driving voltage for causing the first lamp to be driven in synchronization with the first control signal and supplying the first driving voltage to the first lamp; and producing a second driving voltage for causing the second lamp to be driven in synchronization with the second control signal and supplying the second driving voltage to the second lamp.